During the normal vehicle operation, the seat belt rests loosely on a vehicle occupant's body for reasons of comfort, such that in the even of a crash there is a risk that the vehicle occupant will be forwardly displaced in an excessively vigorous manner, despite the blocking of the spool of the automatic seat belt retractor. The layers of belt webbing on the spool of the automatic seat belt retractor are also wound relatively loosely, so that there is also belt slackness which can lead to excessive forward displacement of the vehicle occupant as a result of a film reeling effect, despite blocking of the spool.
To avoid the risk of excessive forward displacement of the vehicle occupant, it is known, prior to blocking of the spool, to eliminate the belt slackness by means of a pretensioner which acts on the seat belt buckle or on the spool of the automatic seat belt retractor.